Since 3/5 of an hour is 36 minutes and the robot can complete 8 tasks in 36 minutes. The remaining minutes are 24 and in 24 minutes the robot can do about 5 tasks so add 5 and 8 to get about 13 tasks in an hour.
Explanation: Just divide 36 by 24 to get 1.5 and divide 8 by 1.5 to get about 5. Then add 8 + 5 to get 13 the final result. So it’s 13 tasks. Sorry if I got it wrong.